Field of the Invention
The invention relates to a cold-rolled flat steel product for deep drawing applications, having a reduced weight as a result of a reduction in density combined with optimized mechanical properties and optimized formability. The invention likewise relates to a method for producing such a flat steel product.
Description of Related Art
Where flat steel products are mentioned here, this means steel strips obtained by rolling operations, steel sheets, and blanks, precut pieces and the like that have been obtained therefrom.
If figures relating to the content of an alloy element are given here in connection with an alloying method, these relate to the weight, unless explicitly stated otherwise.
Especially in the case of flat steel products used in the field of motor vehicle construction, not only the ratio of strength to formability but also physical properties such as stiffness and density are of particular significance with regard to the general aim of weight saving and improvement in the intrinsic frequencies of the respective motor vehicle. Distinct minimization of the density, accompanied by minimization of weight, can be achieved in the case of steels by addition of greater contents of lightweight Al to the alloy. In the case of sufficiently high Al contents, in addition, the initial order phase (K state) or Fe3Al (D03) order phase occurs, and these have particle-hardening, strength-enhancing and ductility-reducing effects.
The application-related advantages of ferritic Fe—Al steels having high Al contents of the kind in question here are opposed by the difficulties in production and processing. Thus, practical experience shows that any non-recrystallized strip core region in the hot strip produced from steels of this kind has to be reduced, since difficulties can otherwise occur in the trimming and in the cold rolling of the hot strip. Furthermore, complex operations are necessary in the prior art in order to avoid anisotropic cold strip properties because of an unsuitable cold strip texture. Anisotropism of this kind is characterized by low r and n values, and entails a low elongation at break. This results in problematic forming and processing characteristics of cold-rolled flat steel products produced from Fe—Al steels having high Al contents.
The problems summarized above increase with rising Al content and therefore limit the reduction in density achievable to date. It is thus considered in industry that Al-containing deep-drawable steels may contain a maximum of 6.5% by weight of Al (see U. Brüx “Tiefziehfähige Eisen-Aluminium-Leichtbaustähle” [Deep-drawable lightweight iron-aluminum steels], Konstruktion April 4, 2002).